Differential plane mirror interferometer

ABSTRACT

A differential plane mirror interferometric comprises a source (10) which emits a light beam containing two linear orthogonally polarized components; a tilted parallel plate (16) having regions of reflection, antireflection and polarization coatings for converting the input beam into two separated, parallel, orthogonally polarized beams; a half-wave retardation plate (29A, 29) located in one of the separated beams for converting said two separated, parallel, orthogonally polarized beams into two separated parallel beams with the same polarization; means including a polarizing beamsplitter (44), for causing each of the separated parallel beams with the same polarization to be reflected twice by one of two plane mirrors (71, 70) to produce two parallel output beams with the same polarization; a half-wave retardation plate (29B, 29) located in one of the separated parallel output beams, with the tilted parallel plate (16) having regions of reflection, antireflection and polarization coatings for converting the two separated parallel orthogonally polarized output beams into a single output beam in which the phase difference between the two polarization components of the single output beam is directly proportional to the optical path length between the two plane mirrors (70, 71); a polarizer (81) for mixing the orthogonal components of the output beam; a photoelectric detector (83) to produce the measurement signal; and an electronic module (90) to indicate the phase difference which is directly proportional to the changes in the optical path length between the two plane mirrors.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of my copending U.S. patentapplication entitled "Differential Plane Mirror Interferometer," filedDec. 19, 1985, and bearing U.S. Ser. No. 810,999, the contents of whichare specifically incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatus for the measurement ofoptical path length changes between two plane mirror surfaces. Moreparticularly, the invention relates to optical apparatus which is usefulfor high accuracy displacement metrology using interferometry.

2. The Prior Art

An interferometer is a basic instrument for most high accuracydisplacement measurements in dilatometry, material stability studies,the machine tool industry, and in the semiconductor fabricationindustry. One type of interferometer representative of the currentstate-of-the-art is the differential plane mirror interferometer whichmeasures the optical path length changes between two external mirrorsand which is described in R. R. Baldwin and G. J. Siddall, "A doublepass attachment for the linear and plane interferometer," Proc. SPIE,Vol. 480, pp. 78-83 (May 1984). A conventional differential plane mirrorinterferometer consists of a fixed plane mirror and a movable planemirror, which form the interferometer cavity, and auxiliary opticalcomponents (retroreflectors, wave plates, mirrors, beamsplitters). Thistype of interferometer has an inherent optical resolution of one quarterof the wavelength of the light used and has particularly high stabilitywhich is necessary for the ever increasing demand for improved accuracy.Thus, it is particularly insensitive to any tilt of the plane mirrorsand motion of the auxiliary optical components.

The conventional differential plane mirror interferometer is, however,overly complicated, requiring many auxiliary optical components therebysubjecting the measurement beams to many reflections. These drawbacksultimately limit the accuracy that can be achieved due to a lowersignal-to-noise ratio in the measurement signal as a result of reducedoptical beam power and polarization leakage.

The present invention retains the basic plane mirror interferometercavity of the conventional differential plane mirror; however, the useof a shear plate in the present invention not only reduces the number ofoptical elements but also reduces the number of reflections by nearly50%. The improvements of the present invention thusly further increasethe accuracy that can be attained with this type of interferometer.

SUMMARY OF THE INVENTION

In accordance with the instant invention, I provide a differential planemirror interferometer system capable of measuring accurately eitherchanges in length or changes in optical length comprising: (1) a sourceof an input beam with two linear orthogonally polarized components whichmay or may not be of the same optical frequency, (2) means, mostpreferably a tilted parallel plate or shear plate with regions ofreflection, antireflection and polarizing coatings, for converting saidinput beam into two separated, parallel, orthogonally polarized beams;(3) means, most preferably a halfwave retardation plate, located in oneof said separated beams, for converting said two separated, parallel,orthogonally polarized beams into two separated, parallel beams with thesame polarization; (4) means, most preferably a polarizing beamsplitter,quarter-wave retardation plate, and retroreflector, for causing each ofsaid separated, parallel beams with the same polarization to bereflected twice by one of two plane mirrors, respectively, to producetwo parallel output beams with the same polarization; (5) mostpreferably a half-wave retardation plate, located in one of saidseparated, parallel output beams for converting said two separated,parallel output bems of the same polarization into two separated,parallel output beams with orthogonal polarization; (6) means, mostpreferably the aforementioned tilted plate with regions of reflection,antireflection and polarizing coatings, for converting said twoseparated, parallel, orthogonally polarized output beams into a singleoutput beam in which the phase difference between the two polarizationcomponents of said single output beam is directly proportional to theoptical path length between said two plane mirrors; (7) means, mostpreferably a polarizer, for mixing said orthogonal components of saidsingle output beam; (8) means, most preferably a photoelectric detector,to produce an electrical measurement signal, and (9) means to extractsaid phase difference from said electrical measurement signal, saidextracted phase difference being proportional to the optical path lengthchanges between the two plane mirrors.

THE DRAWINGS

In the drawings,

FIG. 1 depicts in schematic form one embodiment of the instant inventionwhere all optical beams are in a single plane.

FIG. 2 depicts in schematic form a second embodiment of the instantinvention where the optical beams are not in a single plane.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts in schematic form one embodiment of the instant inventionwhere all optical beams are in a single plane. While the apparatus hasapplication for a wide range of radiation sources, the followingdescription is taken by way of example with respect to an opticalmeasuring system. Light source (10), which most preferably uses a laser,emits input beam (12) which is comprised of two linear orthogonallypolarized components as indicated by the dot and arrow, which may or maynot be of the same optical frequency. If the frequencies are the same,see for example, Downs et al. U.S. Pat. No. 4,360,271 issued Nov. 23,1982. If the frequencies are different, see for example, Bagley et alU.S. Pat. No. 3,458,259 issued July 26, 1969 and commonly owned,copending U.S. patent applications Ser. Nos. 710,859, entitled"Apparatus to Transform a Single Frequency, Linearly Polarized LaserBeam Into a Beam with Two, Orthogonally Polarized Frequencies" filedMar. 12, 1985; 710,928, entitled "Heterodyne Interferometer System",filed Mar. 12, 1985; and 710,927, entitled "Apparatus to Transform aSingle Frequency, Linearly Polarized Laser Beam into a High EfficiencyBeam with Two, Orthogonally Polarized Frequencies", filed Mar. 12, 1985all of which are specifically incorporated by reference herein in theirentirety, in which instant source (10) would provide an electricalreference signal (11), shown by dotted lines in FIG. 1, which wouldcorrespond to the frequency difference between the two stabilizedfrequencies. No such reference signal (11) is provided when the twolinear orthogonally polarized components comprising input beam (12) areof the same optical frequency.

Beam (12) is incident on shear plate (16) which is a tilted parallelplate glass substrate with optically flat surfaces (17) and (18) whichare mutually parallel. The function of tilted parallel plate (16) is tospatially separate the two polarization components using conventionalpolarization techniques. Beam (12) passes through surface (17) to becomebeam (13) which has the same polarization as beam (12). Surface (17) hasan antireflection coating (21A) over the region where beam (12) passesthrough it. Polarizing coating (23A) on surface (18) splits beam (13) sothat one polarized component is transmitted as beam (30) whereas theother orthogonally polarized component is reflected as beam (14). Beam(14) is totally reflected from reflective coating (25A) on surface (17)to become beam (15). Beam (15) passes through surface (18) to becomebeam (31) which has the same polarization as beam (15). Surface (18) hasan anitreflection coating (27A) over the region where beam (15) passesthrough it.

Beam (31) passes through half-wave retardation plate (29A) which rotatesthe linear polarization of beam (31) by 90° so that the resultant beam(33) has the same polarization as beam (30). Beams (30 and (33) enterpolarizing beamsplitter (40) with polarizing coating (42) and aretransmitted as beams (34) and (35) respectively. Beams (34) and (35)pass through quarter-wave retardation plate (44) and are converted intocircularly polarized beams (50) and (51) respectively. Beam (51) isreflected from fixed reference mirror (71) to become beam (51A) whilebeam (50) is reflected from movable mirror (70) affixed to the stagewhose relative position is being measured to become beam (50A). Beams(50A) and (51A) pass back through quarter-wave retardation plate (44)and are converted back into linearly polarized beams which areorthogonally polarized to the original incident beams (34) and (35).Beams (50A) and (51A) are reflected by polarizing coating (42) to becomebeams (52) and (53). Beams (52) and (53) are reflected by retroreflector(45) to become beams (54) and (55). Beams (54) and (55) are reflected bypolarizing coating (42) to become beams (56) and (57). Beams (56) and(57) pass through quarter-wave retardation plate (44) and are convertedinto circularly polarized beams (58) and (59).

Beam (59) is reflected from fixed reference mirror (71) to become beam(59A) while beam (58) is reflected from movable mirror (70) to becomebeam (58A). Beams (58A) and (59A) pass back through quarter-waveretardation plate (44) and are converted back into linearly polarizedbeams which are polarized the same as the original incident beams (34)and (35). Beams (58A) and (59A) are transmitted by polarized coating(42) and leave polarizing beamsplitter (40) as beams (60) and (63).Beams (60) and (63) are mutually parallel by virtue of the inherentoptical properties of retroreflector (45), independent of any tilt thatmay be present between mirrors (70) and (71). Beam (60) passes throughhalf-wave retardation plate (29B) which rotates the linear polarizationof beam (60) by 90° so that resultant beam (62) has a linearpolarization which is orthogonal to beam (63). Beam (62) passes throughsurface (18) to become beam (64) which has the same polarization as beam(62). Surface (18) has an antireflection coating (27B) over the regionwhere beam (62) passes through it. Beam (64) is totally reflected fromreflective coating (25B) to become beam (65). Surface (18) hasreflective coating (25B) over the region where beam (64) intersects it.Beam (65) and (63) are recombined to form beam (66) by polarizingcoating (23B). Surface (17) has polarizing coating (23B) over the regionwhere beams (65) and (63) intersect. Beam (66) passes through surface(17) to become beam (80). Surface (17) has an antireflection coating(21B) over the region where beam (66) passes through it.

Beam (80), like input beam (12), has two orthogonally polarizedcomponents. Each component has traversed exactly the same optical pathlength (through air and glass) except for the optical path, "nl",between mirrors (70) and (71) where "n" is the index of refraction ofthe medium between mirrors (70) and (71) and "l" is the distance betweenmirrors (70) and (71). The optical path length corresponding to thisdistance, "l", results in a phase difference between the twopolarization components of beam (80). Motion of mirror (70) causes thisphase difference to vary. This phase variation is directly proportionalto the distance, "L", moved by mirror (70) for a constant "n" and ismeasured by passing beam (80) through polarizer (81), oriented at 45° toeach polarization component, which mixes the two orthogonally polarizedcomponents in beam (80) to give beam (82). Similarly, If "l" is fixedand "n" varies, then the phase variation is directly proportional to thechange in "n". The interference between the two polarization componentsis detected by photodetector (83) producing electrical signal (85).Electronic module (90) extracts the phase variation from electricalsignal (85). When the two polarization components of beam (12) are ofthe same optical frequency, module (90) does not require referencesignal (11) since there is no corresponding frequency difference, andconventionally extracts the phase variation from signal (85) such as inthe manner described in U.S. Pat. No. 4,360,271. However, when the twopolarization components of beam (12) are of different frequencies, anadditionala sinusoidal electrical reference signal (11) equal infrequency to the difference between the two optical frequencies isrequired by electronic module (90), which reference signal (11), aspreviously mentioned, would be provided from source (10) in whichinstance photodetector (83) would detect the interference between thetwo frequency components as a sinusoidal intensity variation with afrequency approximately equal to the difference frequency between thetwo components of beam (12), such as explained in my copending U.S.patent application Ser. No. 810,999 of which this application is acontinuation-in-part, and module (90) would preferably comprise a phasemeter/accumulator such as described in the aforementioned copending U.S.patent application Ser. No. 710,928. In either event, electronic module(90) provides output (92) which is directly proportional to the changein optical path length between mirrors (70) and (71). This opticalconfiguration is extremely insensitive to measurement error becausechanges in the other optical components, such as those inducedmechanically or thermally, affect both polarization components equallyand therefore have no influence on the measured phase variation (92). Inaddition, environmental effects, such as variations in the refractiveindex of air, can be minimized by placing mirror (71) close to mirror(70) to reduce the optical path length difference between the twopolarization components. It should be noted that half-wave retardationplates (29A) and (29B) could be a single element with a hole in it toallow beam (63) to pass through it unaffected.

FIG. 2 depicts in schematic form a second embodiment of the instantinvention where the optical beams are not in a single plane. Thisconfiguration permits a more compact optical system. The description ofthis figure is identical to FIG. 1 and is numbered correspondingly. Theonly differences are that now coatings (21A) and (21B), (23A) and (23B),(25A) and (25B), and (27A) and (27B) in FIG. 1 become coatings (21),(23), (25), and (27) respectively; and half-wave retardation plates(29A) and (29B) in FIG. 1 become single half-wave retardation plate(29).

Thus, in FIG. 2, light source (10), which is previously mentioned, mostpreferably uses a laser, emits input beam (12) which is comprised of twolinear orthogonally polarized components as indicated by the two arrows,which, again, may or may not be of the same optical frequency. Just aswas mentioned with reference to FIG. 1, when the two linear orthogonallypolarized components of beam (12) differ in frequency, source (10)provides an electrical reference signal (11), shown by dotted lines inFIG. 2, corresponding to this frequency difference, with no suchreference signal (11) being provided when the two linear orthogonallypolarized components comprising input beam (12) are of the same opticalfrequency. Beam (12) is incident on shear plate (16) which is a tiltedglass substrate with optically flat surfaces (17) and (18) which aremutually parallel. The function of tilted parallel plate (16) is tospatially separate the two polarization components using conventionalpolarization techniques. Thus, in the embodiment of FIG. 2, beam (12) isdivided by tilted parallel plate (16), with aid of antireflectioncoatings (21) and (27), polarizing coating (23) and reflective coating(25), to become vertically polarized beam (30) and horizontallypolarized beam (31). Beam (31) passes through the single half-waveretardation plate (29) which rotates the linear polarization of beam(31) by 90° so that resultant beam (33) has the same polarization asbeam (30). Beams (30) and (33) enter polarizing beamsplitter (40) withpolarizing coating (42) and are transmitted as beams (34) and (35)respectively. Beams (34) and (35) pass through quarter-wave retardationplate (44) and are converted into circularly polarized beams (50) and(51) respectively. Beam (51) is reflected from fixed reference mirror(71) to become beam (51A) while beam (50) is reflected from movablemirror (70) affixed to the stage whose relative position is beingmeasured to become (50A). Beams (50A) and (51A) pass back throughquarter-wave retardation plate (44) and are converted back into linearlypolarized beams that are orthogonally polarized to the original incidentbeams (34) and (35). Beams (50A) and 51A) are reflected by polarizingcoating (42), retroreflector (45), and polarizing coating (42) a secondtime to become beams (56) and (57). Beams (56) and (57) pass throughquarterwave retardation plate (44) and are converted into circularlypolarized beams (58) and (59). Beam (59) is reflected from fixedreference mirror (71) to become beam (59A) while beam (58) is reflectedfrom movable mirror (70) to become beam (58A). Beams (58A) and (59A)pass back through quarter-wave retardation plate (44) and are convertedback into linearly polarized beams that are polarized the same as theoriginal incident beams (34) and (35). Beams (58A) and (59A) aretransmitted by polarized coating (42) and leave polarizing beamsplitter(40) as beams (60) and (63). Beams (60) and (63) are mutually parallelby virtue of the inherent optical properties of retroreflector (45),independent of any tilt that may be present between mirrors (70) and(71). Beam (60) passes through the single half-wave retardation plate(29) which rotates the linear polarization of beam (60) by 90° so thatresultant beam (62) has a linear polarization which is orthogonal tobeam (63). Beams (62) and (63) are combined by shear plate (16), withthe aid of antireflection coatings (21) and (27), polarizing coating(23) and reflective coating (25), to become beam (80).

Once again beam (80) in the embodiment of FIG. 2, like input beam (12),has two orthogonally polarized components. Each component, as was truewith the FIG. 1 embodiment, has traversed exactly the same optical pathlength (through air and glass) except for the optical path, "nl",between mirrors (70) and (71) where "n" is the index of refraction ofthe medium between mirrors (70) and (71) and "l" is the distance betweenmirrors (70) and (71). The optical path length corresponding to thisdistance "l", results in a phase difference between the two polarizationcomponents of beam (80). Motion of mirror (70) causes this phasedifference to vary. This phase variation is directly proportional to thedistance, "L", moved by mirror (70) for a constant "n" and is measuredby passing beam (80) through polarizer (81), oriented at 45° to eachpolarization component, which mixes the two orthogonally polarizedcomponents in beam (80) to give beam (82). Similarly, if "l" is fixedand "n" varies, then the phase variation is directly proportional to thechange in "n". As was also true in the FIG. 1 embodiment, theinterference between the two polarization components is detected byphotodetector (83) producing electrical signal (85). Again, as was alsotrue with respect to the FIG. 1 embodiment, when the two polarizationcomponents of beam (12) are of the same optical frequency, module (90)does not require reference signal (11) since there is no correspondingfrequency difference, and conventionally extracts the phase variationfrom signal (85) such as in the manner described in U.S. Pat. No.4,360,271. However, when the two polarization components of beam (12)are of different frequencies, an additional sinusoidal electricalreference signal (11) equal in frequency to the difference between thetwo optical frequencies is required by electronic module (90), whichreference signal (11), as previously mentioned, would be provided fromsource (10) in which instance photodetector (83) would detect theinterference between the two frequency components as a sinusoidalintensity variation with a frequency approximately equal to thedifference frequency between the two components of beam (12), such asexplained in my copending U.S. patent application Ser. No. 810,999 ofwhich this application is a continuation in part, and module (90) wouldpreferably comprise a phase meter/accumulator such as described in theaforementioned copending U.S. patent application Ser. No. 710,928. Ineither event, electronic module (90) provides output (92) which, aspreviously mentioned with respect to the FIG. 1 embodiment, is directlyproportional to the change in optical path length, "nl", between mirrors(70) and (71). Thus, both the FIGS. 1 and 2 embodiments employ opticalconfigurations which are extremely insensitive to measurement errorbecause changes in the other optical components, such as those inducedmechanically or thermally, affect both polarization components equallyand therefore have no influence on the measured phase variation (92). Inaddition, as was previously mentioned with reference to the FIG. 1embodiment, environmental effects, such as variations in the refractiveindex of air, can be minimized by placing mirror (71) close to mirror(70) to reduce the optical path length difference between the twopolarization components.

The principle advantages of the instant invention are: (1) fewer numberof optical components, (2) simpler beam paths, (3) fewer reflections,(4) greater light throughput efficiency, (5) smaller wavefrontdistortion, (6) reduced optical leakage, (7) reduced non-linearityerrors, and (8) lower cost.

While a preferred embodiment of the invention has been disclosed,obviously modification can be made therein, without departing from thescope of the invention as defined in the following claims.

What is claimed is:
 1. A differential plane mirror interferometricsystem comprising: a pair of plane mirrors separable by a variableoptical path length; source means for emitting an input beam comprisingtwo stabilized orthogonally polarized optical frequencies; tiltedparallel plate means optically coupled to said input beam for convertingsaid input beam into two separated parallel orthogonally polarizedbeams; means optically disposed in the path of one of said two separatedparallel orthogonally polarized beams for converting said two separatedparallel orthogonally polarized beams into two separated parallel beamshaving the same polarization; means optically coupled to said twoseparated parallel same polarized beams for causing one of said twoseparated parallel same polarized beams to be reflected twice by one ofsaid pair of plane mirrors and the other of said two separated parallelsame polarized beams to be reflected by the other of said pair of planemirrors to produce two parallel output beams having the samepolarization; means optically disposed in the path of one of said twoseparated same polarized parallel output beams for converting said twoseparated same polarized parallel output beams into two separatedorthogonally polarized parallel output beams; means optically coupled tosaid two separated parallel orthogonally polarized output beams forconverting said two separated parallel orthogonally polarized outputbeams into a single output beam having a pair of orthogonally polarizedfrequency components, with a phase difference therebetween beingdirectly proportional to said variable optical path length between saidpair of plane mirrors; means optically coupled to said single outputbeam for mixing said orthogonally polarized components thereof andproducing an electrical measurement signal therefrom; and meansoperatively connected to said electrical measurement signal forextracting said difference in phase from said electrical measurementsignal, said extracted phase difference being proportional to saidvariable optical path length between said pair of plane mirrors; wherebyan optical configuration extremely insensitive to measurement error andmisalignment is provided for said interferometric system, said tiltedparallel plate means comprising a first set of regions of reflection,antireflection and polarizing coatings, said means for converting saidtwo separated parallel orthogonally polarized output beams into saidsingle output beams comprising said first set of regions and coatings onsaid tilted parallel plate means, said tilted parallel plate meansfurther comprising a second set of regions of reflection, antireflectionand polarization coatings, said second set of regions and coatingscomprising said input beam converting means.
 2. A differential planemirror interferometric system in accordance with claim 1 wherein saidsource means emits said input beam comprising two stabilizedorthogonally polarized optical frequencies which are of the samefrequency value.
 3. A differential plane mirror interferometric systemin accordance with claim 1 wherein said means for converting said twoseparated parallel orthogonally polarized beams into said two samepolarized separated parallel beams comprises a half-wave retardationplate means.
 4. A differential plane mirror interferometric system inaccordance with claim 2 wherein said means for converting said twoseparated parallel orthogonally polarized beams into said two samepolarized separated parallel beams comprises a half-wave retardationplate means.
 5. A differential plane mirror interferometric system inaccordance with claim 1 wherein said means for causing each of saidseparated parallel same polarized beams to be reflected twice by one ofsaid pairs of plane mirrors comprises a polarizing beam splitter meansand a retroreflector means.
 6. A differential plane mirrorinterferometric system in accordance with claim 5 wherein said means forcausing each of said separated parallel same polarized beams to bereflected twice by one of said pairs of plane mirrors further comprisesa quarter-wave retardation plate means.
 7. A differential plane mirrorinterferometric system in accordance with claim 2 wherien said means forcausing each of said separated parallel same polarized beams to bereflected twice by one of said pairs of plane mirrors comprises apolarizing beam splitter means and a retroreflector means.
 8. Adifferential plane mirror interferometric system in accordance withclaim 7 wherein said means for causing each of said separated parallelsame polarized beams to be reflected twice by one of said pairs of planemirrors further comprises a quarter-wave retardation plate means.
 9. Adifferential plane mirror interferometric system in accordance withclaim 1 wherein said means for converting said two separated parallelsame polarized output beams into said two separated parallelorthogonally polarized output beams comprises a half-wave retardationplate means.
 10. A differential plane mirror interferometric system inaccordance with claim 2 wherein said means for converting said twoseparated parallel same polarized output beams into said two separatedparallel orthogonally polarized output beams comprises a half-waveretardation plate means.
 11. A differential plane mirror interferometricsystem in accordance with claim 1 wherein said means for producing saidelectrical measurement signal comprises a polarizer means for mixingsaid single output beam orthogonal components.
 12. A differential planemirror interferometric system in accordacne with claim 11 wherein saidmeans for producing said electrical measurement signal further comprisesa photoelectric detector.
 13. A differential plane mirrorinterferometric system in accordance with claim 2 wherein said means forproducing said electrical measurement signal comprises a polarizer meansfor mixing said single output beam orthogonal components.
 14. Adifferential plane mirror interferometric system in accordance withclaim 13 wherein said means for producing said electrical measurementsignal further comprises a photoelectric detector.
 15. A differentialplane mirror interferometric system in accordance with claim 1 whereinsaid means for producing said electrical measurement signal comprises aphotoelectric detector.
 16. A differential plane mirror interferometricsystem in accordance with claim 2 wherein said means for producing saidelectrical measurement signal comprises a photoelectric detector.
 17. Adifferential plane mirror interferometric system in accordance withclaim 1 wherein said tilted parallel plate means comprises both saidfirst and second set of regions and coatings.
 18. A differential planemirror interferometric system in accordance with claim 1 wherein saidsource means comprises a laser.
 19. A differential plane mirrorinterferometric system in accordance with claim 2 wherein said sourcemeans comprises a laser.
 20. A differential plane mirror interferometricsystem in accordance with claim 17 wherein said tilted parallel platemeans comprises a tilted glass substrate having mutually paralleloptically flat surfaces, with said sets of regions and coatings beingdisposed on said optically flat surfaces.
 21. A differential planemirror interferometric system in accordance with claim 1 wherein saidtilted parallel plate means comprises a tilted glass substrate havingmutually parallel optically flat surfaces, with said sets of regions andcoatings being disposed on said optically flat surfaces.
 22. Adifferential plane mirror interferometric system in accordance withclaim 1 wherein one of said pair of plane mirrors is fixed and comprisesa reference mirror and the other of said pair of plane mirrors isavailable for providing said variable distance between said pair ofseparable plane mirrors.
 23. A differential plane mirror interferometricsystem in accordance with claim 2 wherein one of said pair of planemirrors is fixed and comprises a reference mirror and the other of saidpair of plane mirrors is movable for providing said variable distancebetween said pair of separable plane mirrors.
 24. A differential planemirror interferometric system in accordance with claim 1 wherein all ofsaid beams are in a single plane.
 25. A differential plane mirrorinterferometric system in accordance with claim 24 wherein said sourcemeans comprises a laser.
 26. A differential plane mirror interferometricsystem in accordance with claim 2 wherein all of said beams are in asingle plane.
 27. A differential plane mirror interferometric system inaccordance with claim 26 wherein said source means comprises a laser.28. A differential plane mirrors interferometric system in accordancewith claim 1 wherein all of said beams are optical beams, said opticalbeams being in a given plane.
 29. A differential plane mirrorinterferometric system in accordance with claim 28 wherein said sourcemeans comprises a laser.
 30. A differential plane mirror interferometricsystem in accordance with claim 2 wherein all of said beams are opticalbeams, said optical beams being in a given plane.
 31. A differentialplane mirror interferometric system in accordance with claim 30 whereinsaid source means comprises a laser.
 32. A differential plane mirrorinterferometric system in accordance with claim 1 wherein the distancebetween said pair of plane mirrors is fixed for providing the variationsin the index of refrection of the medium between said pair of planemirrors.
 33. A differential plane mirror interferometric system inaccordance with claim 2 wherein the distance between said pair of planemirrors is fixed for providing the variations in the index of refractionof the medium between said pair of plane mirrors.